THis pist is about making an optical face illusion like this one:
With these two photos:
I started with flipping both photos horizontal, but that is not necessary.
Crop the second photo like this:
Make a selection of the face. Easiest way to do this is with the Magic Wand Tool (W) and Inverse your selection (Select > Inverse)
Now hit Cmd J or Ctrl J on Window to get the selection on his own layer.
Drag the first Photo over this one with the Move Tool (V).
Cmd click on the selection of the Face and then click on Mask icon. (Ctrl Click on Windows).
Now you've get these 3 layers:
The face of the first photo now has a layer. Unlink the mask and use Cmd T or Ctrl T on Windows to make the face bigger so that it fits. Make sure to select the face and not the mask if you do this!
Alignment of the lips and nose is very important to make it look natural.
Now you can tweak it to exactly the image you want to have.